This invention relates to a substrate for a magnetic information storage medium used for an information storage device and, more particularly, to a substrate for a magnetic information storage medium such as a magnetic disk made of a glass-ceramic having an improved surface characteristic including a super flatness suited for a contact recording system and being capable of preventing sticking of a magnetic head to a magnetic information storage medium in a CSS (contact start and stop) type magnetic information storage medium, a magnetic information storage medium formed by subjecting the magnetic information storage medium substrate to a film-forming process and a method for manufacturing the magnetic information storage medium substrate. In this specification and claims, the term "magnetic information storage medium" means a magnetic information storage medium in the form of a disk such as a magnetic disk used as a hard disk in a personal computer and a magnetic information storage disk which can be used in a digital video camera.
There is an increasing tendency for utilizing a personal computer for multiple media purposes and a digital video camera and this tendency necessitates an information storage medium of a larger recording capacity. For this purpose, the bit number and track density of a magnetic information storage medium must be increased and the size of a bit cell must be reduced for increasing a surface recording density. As for a magnetic head, it must be operated in closer proximity to the surface of the magnetic information storage medium in conformity with the reduction of the bit cell. In a case where a magnetic head operates at an extremely low flying height or in a semi-contact state against a magnetic information storage medium, there occur sticking of a magnetic head to a surface of a magnetic information storage medium and accompanying head crash and damage to a magnetic film at the time of start and stop of the magnetic head.
For overcoming such problem, there is an increasing necessity for development of a new technique for start and stop of a magnetic head such as a landing zone system according to which a processing for preventing sticking of a magnetic head is made in a particular portion (i.e., a portion about the inner periphery where no recording or writing of data is made) of a magnetic information storage medium substrate and the magnetic head starts and stops in this particular portion. In the current CSS type information storage device, the magnetic head repeats an operation according to which the magnetic head is in contact with the magnetic disk before starting of operation of the device and is lifted from the magnetic disk when the operation of the device is started. If the surface of contact of the magnetic head with the magnetic disk is a mirror surface, sticking of the magnetic head to the magnetic disk occurs with the result that rotation of the magnetic disk is not started smoothly due to increase in friction and damage to the disk surface occurs. Thus, the magnetic disk is required to satisfy two conflicting demands of realizing lowering of the magnetic head and prevention of sticking of the magnetic head. For satisfying these two conflicting demands, development of the technique of the landing zone system is under way. Further, not only the current fixed type information storage devices, but also a removable information storage device such as a card hard disk drive is being developed for use in, e.g., a digital video camera. From the standpoint of such new technique, characteristics required for a substrate for a magnetic information storage medium substrate are as follows:
(1) In the CSS characteristics in the landing zone type magnetic information storage medium, sticking tends to occur between the head and the magnetic information storage medium due to increase in contact resistance caused by a high speed rotation of the magnetic information storage medium, if the magnetic information storage medium has a smooth surface with surface roughness (Ra) below 50 .ANG. in a landing zone of the medium, i. e., an area in the medium where the magnetic head starts and stops its operation. On the other hand, if the magnetic information storage medium has a rough surface with surface roughness (Ra) above 300 .ANG. in the landing zone, there is a danger of occurrence of crash of the magnetic head. It is necessary, therefore, to control the height of projections or depressions on the surface in the landing zone to 50 .ANG. to 300 .ANG. and also to control an interval of such projections or depressions in the landing zone to 10 .mu.m to 200 .mu.m and the surface roughness (Ra) in the landing zone to 10 .ANG. to 50 .ANG..
(2) Since the amount of lifting of the head tends to decrease to the order of 0.025 .mu.m or below owing to improvement in recording density of the magnetic information storage medium, a data zone on the surface of the magnetic information storage medium should have a surface roughness (Ra) of 3 .ANG.-9 .ANG. which enables the head to maintain this amount of lifting.
(3) By reason of an increased surface recording density, the magnetic information storage medium substrate should not have crystal anisotropy, a foreign matter or other defects and should have a dense, fine and homogeneous texture.
(4) The material of the magnetic information storage medium should have sufficient mechanical strength and hardness for standing a high speed rotation, contact with the head and use as a portable device such as a removable information storage device.
(5) As the surface recording density of a magnetic information storage medium is increased, an extremely fine and precise magnetic film such as a vertical magnetic film is required. For satisfying such requirement, the material of the magnetic information storage medium substrate must not contain Na.sub.2 O, B.sub.2 O.sub.3, OH group and F ingredients in principle because these ingredients cause dispersion of ion of such ingredient during the film forming process with resulting production of coarse magnetic film particles and deterioration in the orientation of the magnetic film particles.
(6) The material of the magnetic information storage medium must have chemical durability against rinsing and etching with various chemicals.
Aluminum alloy has been conventionally used as a material of a magnetic disk substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness. Further, since an aluminum alloy is a soft material, deformation tends to take place so that it cannot cope with the recent requirement for making the magnetic disk thinner and the requirement for high density recording because the disk tends to be deformed by contact with the head with resulting damage to the recorded contents.
As a material for overcoming this problem of the aluminum alloy substrate, known in the art are glass substrates for magnetic disks made of a chemically tempered glass such as a sodium lime glass (SiO.sub.2 --CaO--Na.sub.2 O) and alumino-silicate glass (SiO.sub.2 --Al.sub.2.sub.3 --Na.sub.2 O). This glass substrate, however, has the following disadvantages:
(1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the disk substrate.
(2) For improving the CSS characteristics, the substrate must be subjected to mechanical or chemical processing called texturing. Since a mechanical processing or a thermal processing such as by laser beam cause a cracking or other defects due to distortion in the chemically tempered layer, texturing must be conducted by a chemical etching or a grain growth processing but this prevents mass production of the product at a competitive cost.
(3) Since the Na.sub.2 O ingredient is included as an essential ingredient in the glass, the magnetic film characteristic of the glass is deteriorated with the result that the substrate cannot cope with the requirement for increasing the surface recording density.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some substrate made of glass-ceramics. For example, Japanese Patent Application Laid-open No.6-329440 discloses a glass-ceramic of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system which includes lithium disilicate (li.sub.2 O 2SiO.sub.2) and alpha-quartz (alpha-SiO.sub.2) as predominant crystal phases. This glass-ceramic is an excellent material in that, by controlling the grain size of globular grains of alpha-quartz, the conventional mechanical texturing or chemcial texturing becomes unnecessary and surface roughness (Ra) of a polished surface can be controlled within a range from 15 .ANG. to 30 .ANG.. This glass-ceramic cannot cope with the above described target surface roughness (Ra) of 3 .ANG. to 9 .ANG. and also cannot sufficiently cope with the above described tendency to lowering of the amount of lifting of a magnetic head necessitated by rapid increase in the recording capacity. Besides, in this glass-ceramic, no discussion about the landing zone to be described later in this specification is made at all.
Japanese Patent Application Laid-open No.7-169048 discloses a photo-sensitive glass-ceramic of a SiO.sub.2 --Li.sub.2 O system including Au and Ag as photo-sensitive metals characterized in that a data zone and a landing zone are formed on the surface of a magnetic disk substrate. A predominant crystal phase of this glass-ceramic is lithium silicate (Li.sub.2 O. SiO.sub.2) and/or lithium disilicate (Li.sub.2 O.2SiO.sub.2). In case lithium silicate (Li.sub.2 O. SiO.sub.2) is used, the glass-ceramic has a poor chemical durability so that it has a serious practical problem. Further, in forming of the landing zone, a part of the substrate (i.e., landing zone) is crystallized and is subjected to chemical etching by using 6% solution of HF. However, forming of the substrate with an uncrystallized part and a crystallized part makes the substrate instable mechanically as well chemically. As for chemical etching by HF solution, it is difficult to control concentration of the HF solution because of evaporation and other reasons so that this method is not suitable for a large scale production of products.
Several methods are known for forming a landing zone and a data zone on the surface of a magnetic disk substrate. For example, Japanese Patent Application Laid-open No.6-290452 discloses a method for forming a landing zone on a carbon substrate by a pulsed laser having a wavelength of 523 nm. In this case, however, there are the following problems:
(1) A carbon substrate is formed by pressing at a high pressure and burning at a high temperature of about 2600.degree. C. with resulting difficulty in a large scale production at a low cost.
(2) A carbon substrate has a high surface hardness and, therefore, processing of end portions and precision polishing of the surface are difficult with resulting difficulty in a large scale production at a low cost.
(3) The forming of the landing zone utilizes oxidization and evaporation of carbon by the pulse laser. Since carbon is a material which causes a very strong thermal oxidization reaction, the formed landing zone becomes instable and thus it poses a serious problem in reproduceability.
Japanese Patent Application Laid-open No.7-65359 and U.S. Pat. No.5,062,021 disclose a method for forming a landing zone on an aluminum alloy by a pulsed laser. The aluminum alloy has the above described problems. Besides, the surface of the substrate after irradiation of laser beam tends to have a defect due to oxidization of a molten portion and remaining of splash of molten metal on the surface.
It is, therefore, an object of the present invention to provide a glass-ceramic substrate for a magnetic information storage medium which has eliminated the above described disadvantages of the prior art magnetic disk substrates and is capable of stably lifting a magnetic head in a landing zone where the magnetic head performs the CSS operation and is also capable of reducing the amount of lifting of the magnetic head in a data zone for realizing a high recording density of the data zone.
It is another object of the invention to provide a magnetic information storage medium formed by subjecting the magnetic information storage medium substrate to a film forming process.
It is another object of the invention to provide a method for manufacturing the glass-ceramic substrate for a magnetic information storage medium.